Hollow concrete blocks are used extensively throughout the country in masonry construction of buildings due to their special characteristics. Masonry units are fireproof, durable, and long lasting. They have a high degree of compressive strength that is important when they are used to form masonry walls carrying substantial roof loads. Thus the importance of concrete blocks in the construction industry cannot be overstressed. Such demands for this particular unit of construction material has required these blocks to be manufactured in great quantities and in ever increasing speeds.
Basically, concrete blocks are made from a mixture of relatively dry concrete, aggregates, and water. They are compacted in molds typically comprising a steel box without a top or bottom and with centrally located rigidly attached cores. The mold or core box holds a number of cores and is provided with separation plates to form a corresponding number of concrete blocks. The metal cores are releasably attached to the core box so they can be easily replaced when worn after continuous use. After installation in a blockmaking machine, the core box is lowered onto a steel plate which serves as the bottom of the mold. Then the core box is filled with the concrete mixture and the core box is vibrated until the concrete is compacted enough to hold its shape. Then the core box is removed, leaving the concrete blocks resting on the steel plate. Finally the steel plate with the concrete blocks are moved onto a conveyer belt to be cured under controlled temperature and moisture conditions. This manufacturing process is repeated when another steel plate is fed into the blockmaking machine under the core box to repeat the cycle.
The speed of this operation and the forces required to lift the cores and the core box out of the concrete blocks exert great pressures on the walls of the cores, so that in the course of time after being used in the manufacture of many concrete blocks the walls of the core start to deform. This deformation causes the concrete mixture to interact with the deformations in the walls in such a way that the forces required to lift the core box and cores out of the concrete becomes greatly magnified. This in turn increases the pressure exerted by the concrete on the walls of the core so that once deformation of the walls of the core begin, the rate of deformation increases more and more rapidly. Finally the force required to lift the core box and cores out of the molded concrete blocks becomes so great that the deformed cores have to be replaced. Up to the present, this occurs when the cores have been used to manufacture between 40 to 70 thousand concrete blocks, the variation in number being dependent on the abrasiveness of the aggregates in the concrete mixture.
Heretofore, cores used in the manufacture of concrete blocks were made from steel plates. Initially two steel plates of a predetermined size and thickness were each bent so they were generally U-shaped in cross section. Each section comprised a web portion and leg flanges which were generally transverse to the web portion. The web portion (in this particular embodiment) formed the side walls of the core. The distal edges of the leg flanges on each plate were welded together to form the side walls of a generally tubular steel box which was rectangular in cross section. Then a steel plate, cut to the proper size, was welded to the ends of edges of the end walls and the side walls formed by the welded leg flanges of each plate to form the top wall of the core. However the bending of the steel plates and the welding together of the distal edges of the leg flanges of each plate introduced deformations in the surface of the core. These deformations, for the reasons described below greatly shortened the life of the core.
Valve openings were formed in the top wall and bosses were welded to the side wall. Holes were drilled through the bosses forming valve guides extending in a direction transverse to the top wall. Spring loaded valves, each including a valve stem were mounted so the valve stem was slidably mounted in the valve guides. The valves were sized to close off openings in the top wall. The length of the valve stem was such that when the cores and the core box were lowered onto a steel plate, the ends of the valve stems engaged the steel plate. This is what pushed the valve stems upward so the valves closed the openings in the top wall. This prevented the concrete mixture being poured into the core box from entering into the interior of the cores. When the core box and the core assembly were lifted off the concrete blocks, the spring loaded valves retracted from the openings, thus preventing suction from drawing the concrete into the interior of the core cavity which would deform the concrete blocks and damage the cores.
In the present state of the art, the core boxes are constructed from steel plates welded together. The welding operation causes some deformation or warping in the surface of the walls of the core. However, it was not previously realized that even small deformations in the surface of the walls of the core greatly magnify the force required to withdraw the cores out of the molded blocks and in addition greatly magnify the pressures exerted on the walls of the core. In addition it was not realized how fast the deformations in the walls of the cores grew because of the abrasiveness of the concrete, thereby shortening the useful work life of the core.
When the cores have to be replaced, the blockmaking machinery is stopped, causing a decrease in production, along with the expense of replacing the cores. It is evident that if the onset of the deformations could be delayed the useful life of the cores would be greatly prolonged.
What is needed therefore and comprises an important object of this invention is to provide a core which is formed so it is more resistant to the onset of deformations in the walls of the core thus greatly increasing the useful work life of the core.